没有弹出网页 ，请问错误在哪啊
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>window对象</title>
<script type="text/javascript">
function aaa
{
alert("欢迎来到慕课网");
window.open('http://www.imooc.com','_blank','width=600 height=400');
}
</script>
</head>
<body>
<form>
<input type="button" value="点击我，打开新窗口" onclick="aaa()">
</form>
</body>
</html>
